The purpose of this study is to look at the level of depo-medroxyprogesterone acetate (DMPA or Depo-Provera) in the blood to see if is affected by certain anti-HIV drugs (nelfinavir [NFV], efavirenz [EFV], indinavir [IDV] in combination with ritonavir [RTV], and nevirapine [NVP]). This study will also look at the levels of these anti-HIV drugs to see if they are affected by DMPA.